display: Relocate get_pname() to reduce plugin duplication

display: Relocate get_pname() to reduce plugin duplication

Remove and redefine get_pname function in plugins/display/core.c

This function is added to below display-misc.
void display_misc_get_process_name(pid_t pid, char *pname)
-> This function gets process name by pid parameter.

During display plugins refactoring, this function will be relocated.
To use macro value in poll.h, poll.h is relocated to core from plugin-common.
All plugin-common files will be moved to display core during refactoring process.

Change-Id: Iaaa32254bd93ee37f7bf5bfe5edafac4beb5ab83
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>