drm/exynos: dsi: fix workaround for the legacy clock name

drm/exynos: dsi: fix workaround for the legacy clock name

Writing to the built-in strings arrays doesn't work if driver is loaded
as kernel module. This is also considered as a bad pattern. Fix this by
adding a call to clk_get() with legacy clock name. This fixes following
kernel oops if driver is loaded as module:

Unable to handle kernel paging request at virtual address bf047978
 pgd = (ptrval)
 [bf047978] *pgd=59344811, *pte=5903c6df, *ppte=5903c65f
 Internal error: Oops: 80f [#1] SMP ARM
 Modules linked in: mc exynosdrm(+) analogix_dp rtc_s3c exynos_ppmu i2c_gpio
 CPU: 1 PID: 212 Comm: systemd-udevd Not tainted 5.6.0-rc2-next-20200219 #326
 videodev: Linux video capture interface: v2.00
 Hardware name: Samsung Exynos (Flattened Device Tree)
 PC is at exynos_dsi_probe+0x1f0/0x384 [exynosdrm]
 LR is at exynos_dsi_probe+0x1dc/0x384 [exynosdrm]
 ...
 Process systemd-udevd (pid: 212, stack limit = 0x(ptrval))
 ...
 [<bf03cf14>] (exynos_dsi_probe [exynosdrm]) from [<c09b1ca0>] (platform_drv_probe+0x6c/0xa4)
 [<c09b1ca0>] (platform_drv_probe) from [<c09afcb8>] (really_probe+0x210/0x350)
 [<c09afcb8>] (really_probe) from [<c09aff74>] (driver_probe_device+0x60/0x1a0)
 [<c09aff74>] (driver_probe_device) from [<c09b0254>] (device_driver_attach+0x58/0x60)
 [<c09b0254>] (device_driver_attach) from [<c09b02dc>] (__driver_attach+0x80/0xbc)
 [<c09b02dc>] (__driver_attach) from [<c09ade00>] (bus_for_each_dev+0x68/0xb4)
 [<c09ade00>] (bus_for_each_dev) from [<c09aefd8>] (bus_add_driver+0x130/0x1e8)
 [<c09aefd8>] (bus_add_driver) from [<c09b0d64>] (driver_register+0x78/0x110)
 [<c09b0d64>] (driver_register) from [<bf038558>] (exynos_drm_init+0xe8/0x11c [exynosdrm])
 [<bf038558>] (exynos_drm_init [exynosdrm]) from [<c0302fa8>] (do_one_initcall+0x50/0x220)
 [<c0302fa8>] (do_one_initcall) from [<c03dd02c>] (do_init_module+0x60/0x210)
 [<c03dd02c>] (do_init_module) from [<c03dbf44>] (load_module+0x1c0c/0x2310)
 [<c03dbf44>] (load_module) from [<c03dc85c>] (sys_finit_module+0xac/0xbc)
 [<c03dc85c>] (sys_finit_module) from [<c0301000>] (ret_fast_syscall+0x0/0x54)
 Exception stack(0xd979bfa8 to 0xd979bff0)
 ...
 ---[ end trace db16efe05faab470 ]---

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>