drm/amdgpu: Fix style issues in amdgpu_gem.c

Fixes the following to align to linux coding style:

WARNING: braces {} are not necessary for any arm of this statement
WARNING: Missing a blank line after declarations
ERROR: space prohibited before that close parenthesis ')'
WARNING: unnecessary whitespace before a quoted newline
WARNING: %LX is non-standard C, use %llX

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>